Remove the application type assertion from `Context::isServerCompletelyStarted`

The flag this predicate returns is only ever set by `programs/server`, so in
every other application the assertion was the only thing the call did: debug and
sanitizer builds aborted where a release build returns `false` and carries on.
Applications other than the server reach it through `Context::getZooKeeper` when
they replace a closed session, and through `TransactionLog::removeOldEntries`
after a transaction commits.

Reading the flag from a non-server is reachable code, so allow it instead of
adding a check around every caller, and drop the two guards added earlier in
this branch. The assertion in `Context::setServerCompletelyStarted` stays:
writing the flag outside a server would be a real error.